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New Scientist Live

New sky survey shows that dark energy may one day tear us apart

Measurements reveal a mismatch

Reidar Hahn, Fermilab

By Shannon Hall

The fate of the universe just became a little less certain. That’s due to a disagreement between a map of the early universe and a new map of today’s universe. If the mismatch stands the test of future measurements, we might have to rewrite physics. But that is a pretty big if.

The new results, which are part of the ongoing Dark Energy Survey (DES), charted the distribution of matter across 26 million galaxies in a large swathe of the southern sky.

“This is one of the most powerful pictures of the universe today that we’ve ever had,” says Daniel Scolnic at the University of Chicago, who is a part of the 400-person DES collaboration but wasn’t involved in this work.

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It is so powerful because knowing the distribution, or clumpiness, of galaxies helps us better understand the cosmic game of tug of war as dark energy – a mysterious force that causes the universe to accelerate – pulls each galaxy apart, and dark matter – a theoretical but still unseen form of matter – pushes each galaxy together.

Cosmic selfie

These results are based on data from the 4-metre Victor M. Blanco Telescope’s first observing season for the DES, which lasted a mere six months. The survey is slated to run for five years, meaning that DES’s measurements can only improve. But its first results have already revealed a tension with earlier findings.

In 2013, astronomers revealed the results of charting the universe’s dark contents across the early cosmos – 380,000 years after the big bang, to be exact – with the help of the Planck satellite.

“We’ve had really good baby pictures of the universe and now, with this, it’s like the first time we’ve had really good selfies,” Scolnic says.

Comparing the two allows us to piece together how the universe evolved from its early state to the present – and make predictions about the future. Many astronomers believe that dark energy is a constant force and didn’t think these results would change over time. DES’s first findings, however, might suggest otherwise.

“If [the two different answers] don’t go away, we’re seeing the first signs of what could be a very serious problem in the cosmological model,” says David Spergel at Princeton University.

Balancing act

Astronomers are hesitant to make too grandiose a claim based on the first data set. Statistically speaking, there’s only a slight tension between the results relating to today’s universe and the early universe. But Scolnic points out that this tension is one in a line of many tensions. Results from the South Pole Telescope, for example, similarly disagree with Planck.

“It’s hard to believe that this is a coincidence,” Scolnic says. And if it isn’t a coincidence, the results could mean that dark energy actually grows stronger with time – an idea that has wild implications for our future.

Before, we assumed that although the universe would continue to expand, galaxies would remain forever bound. But should dark energy increase with time, then one day galaxies – plus stars, planets and even the atoms within us – will also expand until they are torn to shreds.

“That would be a disturbing conclusion,” Spergel says. But he advises against panicking just yet.

The latest mismatch could mean that one or both of the measurements is wrong. If so, it might disappear with more data. Then again, it might not. And the fate of the universe hangs in the balance.